The present invention relates generally to pneumatic tire carcasses, and more particularly to a breaker-functioning tape for reinforcing a radial ply tire carcass capable of being constructed in a single stage; namely, in one stage, on a single drum, in which a breaker assembly and a tread are applied to the carcass plies before the unit is converted from a substantially cylindrical condition to a substantially toroidal condition.
Radial ply tire manufacturers in increasing numbers are now attempting to build radial ply tires in a single stage to eliminate the need for the additional "second stage" equipment and labor typically required for such tire construction. In this regard, conventional procedure for constructing radial ply tires includes a "first stage" in which the carcass or body plies are wound on a substantially cylindrical building drum. Thereafter, among other things, beads are applied and anchored to the opposite end portions of the cylindrically arranged body plies, and the entire unit (or "first stage carcass") is transformed into a cylindrical form. The cylindrical "first stage carcass" is then transferred to conventional "second stage" equipment at which the cylindrical form is shaped to a toroidal shape and where breaker plies and tread are applied to it for transformation into a "second stage carcass" ready for vulcanization. The additional equipment and labor necessary for effecting the "second stage" as an independent operation is excessively costly not only with respect to equipment and labor, but also with respect to time and space necessary for storing the additional equipment. Clearly, the conventional "two stage" radial ply tire constructing operations are not entirely desirable.
In order to eliminate the "second stage", radial ply tire manufacturers have been applying the breaker plies and tread to the radial ply carcass or body plies in much the same way and on equipment much the same as for building conventional bias ply tires; namely, before the cylindrical carcass is transformed into a toroid. However, such manufacturers hae encountered the difficulty of using breaker plies whose substantially inextensible cords from a high bias angle (in excess of 35.degree.) with a median equatorial plane of the carcass, and thus readily pantograph to accommodate the substantial expansion of the carcass into a toroid. A high bias angle provides for a satisfactory lateral stiffness in the breaker plies, but at the cost of an unsatisfactory or insufficient amount of circumferential stiffness. An insufficient amount of circumferential stiffness will result in an unacceptably high rate of tread wear.
It has been proposed, for example, in conjunction with the disclosure in U.S. Pat. No. 3,900,062, issued on Aug. 19, 1975, the subject matter of which is hereby incorporated by reference herein, to wind in helical fashion a knitted tape, capable of undergoing only a prescribed limited amount of expansion, around the breaker plies to reinforce or supplement the latter with the necessary amount of circumferential stiffness. The knitted tape is provided with a plurality of side by side, substantially inextensible, rubbered encapsulated, cords that are crimped with sinusoidal undulations and releasably held as a unit adjacent to one another by, for example, a frangible cord knitted into and through the undulations.
One difficulty associated with the use of such a breaker-functioning knitted tape is its relatively high expense to fabricate. The inextensible cords are encapsulated in a sheath of rubber by an extrusion process that is excessively costly, and then knitted to one another, compounding the cost.
Another difficulty associated with the use of any breaker-functioning tape is the manner by which it is only helically wound, near zero degrees, around the underlying carcass construction. A tire completed in such a manner sometimes displays uneven tread wear in much the same way that would be displayed by the tread of a tire on a vehicle requiring an adjustment in alignment. It is theorized that the helical tape turns act something akin to screw threads forcing the tire components to shift or twist slightly relative to one another as the tire turns. This causes certain portions of the tread at the "footprint" or tread/road interface to shimmy or pivotally skid about an imaginary vertical axis normal to the footprint, thereby effecting concentrated high local stress and tread abrasion.